In recent times, the technology for filtering particulates from gases has become quite sophisticated in both commonplace applications such as consumer oriented vacuum cleaning of dirt and dust as well as very demanding industrial applications such as removal from gases of specific particle size fractions of wide varieties of contaminants including from inert to biochemically sensitive, among others. It is now well appreciated that the contaminating particulates in a gas stream can have a wide variety of sizes, geometric shapes, e.g., elongated and spherical, and chemical and physical compositions, e.g., odor-free and odoremitting particles.
Consequently, filtration technology has evolved to provide filter media which are adapted to optimally filter specific fractions of the contaminating particulates. Also, this technology has developed techniques for optimizing various performance characteristics of filters such as maintaining low pressure drop across the filter and increasing the filter service life so as to extend the length of time between filter element replacements.
The traditional approach to achieving these objectives has been to provide a multilayer filter medium composed of separate, individually designed layers which are each intended to accomplish primarily one, and sometimes several specific filter functions. For example, a very open, porous and thin scrim is often used to protect underlying filter layers from abrasion by fast moving, large and hard particles; a porous and bulky layer is typically used to capture substantial amounts of chiefly large particles, and an ultrafine diameter filament, low porosity layer is usually prescribed for removing the smallest particles to increase filtration efficiency. From the many choices available, separate filter layers are selected and combined in a preselected sequence then assembled as a group to form a multilayer, and therefore multifunctional filter. The one or more adjacent layers can be bonded to each other or the layers can be unbonded. Optionally, the individual layers can be sandwiched between covers, typically of paper, for structural integrity and ease of handling.
A drawback of the aforementioned multilayer system of constructing multifunctional filters is that there is repetitive processing of the filter media which can be excessive. That is, the filter material in a given layer is first processed to form the individual layer, then it is processed to assemble that layer in the multilayer filter. Each step adds to the compaction and cover, if ever slight, of the final filter product. This tends to raise the pressure drop through the filter and reduce dust holding capacity, thereby limiting service life.
WO 01/03802 discloses a composite filter comprising at least one non-prebonded upstream tier and one non-prebonded downstream tier. However, as will be shown in detail below (FIG. 2), in this composite filter a relatively high pressure drop across the composite filter occurs. Further, also the service life time of this filter is low.
In view of this, the objective problem underlying the invention is to provide a composite filter in which the pressure drop across the filter is maintained low and which has a long service life time.